It is well known to those skilled in the art that a representative para-dialkyl-substituted benzene is para-xylene. The production of para-xylene is typically performed by methylation or disproportionation of toluene over a catalyst under conversion conditions. Examples include the reaction of toluene with methanol as described in Chen et al., Journal of Amer. Chem. Soc., 101, 6783 (1979), and toluene disproportionation, as described by Pines in "The Chemistry of Catalytic Hydrocarbon Conversions", Academic Press, N.Y., 1981, p. 72. Such methods typically result in the production of a mixture including para-xylene, ortho-xylene, and meta-xylene. Depending on the degree of selectivity of the catalyst for para-xylene (para-selectivity) and the reaction conditions, different percentages of para-xylene are obtained. The yield, i.e., the amounts of xylene produced as a proportion of the feedstock, is also effected by the catalyst and the reaction conditions.
Economically, para-xylene is a more attractive product than ortho-xylene and meta-xylene. Unfortunately, the separation of para-xylene from ortho-xylene and meta-xylene is difficult and expensive. Accordingly, because of the large scale economics involved in the production processes, even a small improvement in the para-xylene selectivity of the process can add millions of dollars to the bottom line. Consequently, methods for increasing the para-selectivity of zeolite catalysts are of great importance.